SkrapIron
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Understanding Electric Motor Basics.
By SkrapIron and AS-EE
It happens every time I complete a run with my electric powered truck at the track. Almost all of the other trucks and buggies that are run there are nitro powered. I constantly hear comments like, “Man, that thing’s fast, for an electric.” “It’s so quiet.”
It really doesn’t surprise me any more, since there is such a grave misunderstanding of just how electric motors work. Most enthusiasts have a good understanding of the internal combustion engine, but to those same people an electric motor is an enigma.
In our hobby, we use a permanent magnet direct current (DC) motor. All of these motors operate from the DC voltage supplied by a battery pack. The battery chemistry can vary greatly, and what type is chosen can dramatically affect the performance of the motor. At the core of these motors lie magnets that are typically made of iron-ferrite or, less often, a rare earth material such as neodymium.
Lower-cost iron-ferrite motors usually employ oiled bronze bushings to support the ends of the armature shaft and stamped-metal can housing. Better-quality motors use stronger but more expensive rare earth magnets and almost always have ball bearings to support the shaft in machined housings. Motors will vary greatly in their physical size as well as their capacities, so selection of a proper motor for your application is critical.
The DC motor is broken into 2 distinct components. The stationary parts of the motor, collectively called the "stator," include the magnets, brushes, brush hood, and springs. The rotating portion or "rotor" includes the commutator plates, armature and windings. The motor's job is to convert stored electrical energy into mechanical energy. It does so through the process called commutation. Commutation occurs when a portion of the windings on the armature are energized in any one position. As the motor’s position in the magnetic field changes, the brushes connect to different windings through the commutator plates. The brush motor is designed so that the optimal windings are energized in every position.
Brush motors are most often identified using the number of turns and winds with which it is constructed.
A turn refers to a complete wrap of a coil of wire around an armature arm (15 turns means 15 loops of 1 wire on each arm). Higher number turns will be slower because more copper coils are exposed to the magnetic flux lines of the stator magnets. This will cause a greater amount of counter-voltage to be induced when the armature "cuts" through the magnetic lines per unit time. Counter-voltage then subtracts from supply voltage in the armature coils, and this will in turn cause your armature coils to have less current flowing through them which results in a slowing of rotational speed. A lower number of turns will be faster, since fewer copper coils are exposed to the flux lines of the stator magnets. It is in these applications that higher flux density magnets, such as neodymium, can be used
A wind refers to the number of strands of wire, wrapped around the armature, in 1 turn. In general, winds with fewer wires give greater starting torque and better acceleration, but lower top end. Conversely, higher winds with more wires will have less starting torque, but higher top end.
A brushless DC motor functions a bit differently than its brushed counterpart. In a brushless motor, the permanent magnets are mounted on the rotor. The windings are stationary and attached to the motor's outer case. Since it is now the magnet that is turning, instead of the windings, the commutation must be done electronically rather than mechanically. An integrated sensor circuit in the motor, along with the microprocessor in the Electronic Speed Control, controls commutation. This control is delivered by calculating the rotor’s position, and determining how to channel the current to supply the requested torque with minimal current. These sensor based motors provide the smoothest and quickest delivery of torque, but are limited in their RPM potential. Several DC brushless motors forgo the integrated sensor circuit in favor of a 6 step drive. Six-step drives channel current into only two windings at any one time. This simplifies the design and construction of the drive. However, the torque produced by six-step drives has more ripple and is produced less efficiently, compared to sensor based brushless motors.
Brushless, sensorless motors with three connections are in fact, not DC motors at all. They are actually permanent magnet synchronous AC, 3-phase motors. The ESCs that control them, have three distinct semi sinusoidal waveforms (not pure sinewave AC) that come in at different times (or degrees) which causes the rotor to rotate with the changing (alternating) magnetic fields of the stator. While this is still not as smooth in operation as a sensor-based motor, it does allow for tremendously greater power, and a much higher operating RPM.
Brushless motors are inherently more efficient than brushed motors for multiple reasons. Since there is no mechanical commutation, there is no wasted energy from friction between the brushes and commutator. There is also no loss in efficiency due to the build up of contamination on the commutator. Additionally, because the windings are physically mounted to the outer case, heat is more efficiently drawn away from the motor.
For any motor, either brushed or brushless, the voltage that is supplied to the windings controls the speed of the rotor. This supply is referred to as the voltage constant, which is represented with the symbol Kv. Kv is the rpm per volt that the motor will produce. (Kv x Volts = RPM) The higher the Kv, the faster the motor shaft will turn for each volt applied. But judging the performance of a motor solely based on its voltage constant (Kv), is a mistake.
Along with the Kv of the motor, there is something else to consider. Every motor has a torque constant (Kt). Kt is the amount of torque the motor can deliver to the pinion shaft, and is rated in either ounce/inches or Newton millimeters per amp of current. What is difficult to understand at first is that the Kt is inversely proportional to the Kv of the motor. That means, as the Kv of the motor increases, the Kt decreases. It therefore stands to reason that a high Kv motor cannot deliver as much torque as a similar-size lower Kv motor.
When a DC motor is energized, it draws a large initial surge of current. The surge is caused because the motor, when it is turning, also acts as a generator. The generated voltage is directly proportional to the speed of the motor. The current through the motor is controlled by the difference between the battery voltage and the motor's generated voltage, otherwise called back EMF. When power is first applied to the motor, there is no back EMF. That means that the current is controlled only by the battery voltage, battery internal resistance, motor internal resistance and the battery leads. Without any back EMF the motor, as it starts to turn, the motor draws the large surge current. This surge of current is what generates huge amounts of initial torque. As the current flow evens out, with the back EMF, the torque curve falls off proportionally.
< Message edited by SkrapIron -- 9/2/2005 2:38 AM >
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All Forums >> RC Cars, Buggies, Trucks, Tanks and more >> RC Electric Off-Road Trucks, Buggies, Truggies and more >> Understanding Motor Basics 
Understanding Motor Basics - 
RE: Understanding Motor Basics - 
Good job Skrap! I assume this intended for a sticky, right? 
